The present invention relates to integrated circuits and, more particularly, to biasing the body of a functional unit block in an integrated circuit.
An integrated circuit, such as a processor, includes a large number of transistors, and many of the transistors are intended to have identical operating parameters. For example, the input transistors of the logic gates in a processor are intended to have identical threshold voltages and leakage currents. Unfortunately, manufacturing process variations, such as random dopant fluctuations, over the area of a die on which an integrated circuit is fabricated, can cause transistors fabricated in different areas of the die to have different threshold voltages and leakage currents. Transistors that have different threshold voltages have different maximum operating frequencies, and transistors that have different leakage currents consume different amounts of power.
This transistor-to-transistor variation causes several inefficiencies during the operation of an integrated circuit. First, some transistors in the integrated circuit have a lower than intended threshold voltage. When operated at the integrated circuit""s target operating frequency, these transistors have a larger leakage current than the transistors that have the intended threshold voltage. The larger leakage current causes the integrated circuit to consume more power than necessary. Second, some transistors in the integrated circuit have a higher than intended threshold voltage. These transistors have a maximum operating frequency that is less than the integrated circuit""s target operating frequency, which prevents the integrated circuit from operating at its target operating frequency.
Transistor operating frequencies can be made more uniform and leakage currents can be reduced in an integrated circuit by applying a bias voltage to the body of each transistor or group of transistors in the integrated circuit. The bias voltage can be chosen to increase the threshold voltage of each transistor, which decreases the maximum operating frequency of each transistor and decreases the leakage current in each transistor, or the bias voltage can be chosen to decrease the threshold voltage of each transistor, which increases the maximum operating frequency of each transistor. To identify the proper bias voltage for a particular group of transistors in an integrated circuit, a separate test path that duplicates the critical path (the path that must operate the fastest) for a particular group of transistors is fabricated on the die. A bias voltage is identified that causes the test path to operate correctly at the integrated circuit""s target operating frequency. The identified bias voltage is then applied to all transistors in the selected group of transistors to reduce leakage current power consumption in the selected group of transistors and to prepare the selected group of transistors to operate at the integrated circuit""s target operating frequency. Unfortunately, identifying a single critical path in an integrated circuit is difficult because most integrated circuits have multiple paths that are intended to operate at the same maximum frequency, and manufacturing a separate test path on a die wastes valuable die real estate.
For these and other reasons there is a need for the present invention.